This invention relates to the finishing of the airseal portions of gas turbine components and more particularly to the repair and restoration of gas turbine blade and vane airseals which are worn or damaged.
Gas turbine airfoils, such as turbine blades, are adapted to channel quantities of gases at high pressure, and often high temperature. In many designs, a multiplicity of blades having tip shrouds in communication with each other are attached around the periphery of a disk which is adapted to rotate within the enclosed duct of a gas turbine. A seal is needed to prevent the passage of gases between the duct and the circumferential ring which is formed by the mating shrouds of the blades. This is conveniently accomplished by the use of circumferential fins running along the ring. The fins are created by mating segments, or airseals, on each of the individual components' shrouds. By necessity of design, airseals are relatively thin, and therefore, they are delicate. Due to normal service experience and unusual incidents, airseals will become worn or damaged. Since gas turbine airfoils are often made of cast superalloys and other advanced materials which are difficult to fabricate, their repair often represents some difficulty.
When new parts are fabricated, they are processed in relatively large quantity lot sizes. Typically, in the sequence of operations which form an airseal in a finished turbine airfoil, a quantity of airfoils will be assembled in a fixture much like the disk in which they will serve. Then the circumferential fins are machined on the assembly using a large rotary grinder. When disassembled, an airseal will be found on each part. However, such techniques are not economical for the typical service center which must repair small lot sizes of a variety of different configurations. Attempts have been made to simplify the process. Generally, the process used heretofore has comprised putting airfoils in a fixture and using a surface grinder to remove the old airseal; then a new piece of material is welded to the stub; then any surplus weldment material overhanging the ends of the old airseal stub was removed to permit use of one of the two following techniques: either the parts were assembled in a fixture suited to holding a full wheel of parts, such as is used for the original parts, or the parts were placed individually in separated fashion in a rotary fixture suited to hold a reduced number of parts; then they were placed in a rotary grinder and the circumferential surface of the airseals ground. In both instances, the airseal portions of the airfoils were mounted at a distance from a center of rotation which was comparable to that of a turbine engine. Since the diameters of modern engines are commonly more than half a meter, this required relatively large machine tools. Furthermore, setup time was rather lengthy when changing from one part number to another. Normal checking and dressing of the grinding wheel dimension was necessary to assure dimensional accuracy. Labor costs were relatively high.
A machine for avoiding some of the disadvantages of the aforementioned process is described in DeMusis, U.S. Pat. No. 3,969,848. The DeMusis machine has a shoe adapted to contour the belt in its direction of motion. The airseal length is aligned with the travel direction and the force of the airseal on the belt is what forms the belt to the shoe contour. It is evident the belt will resist this deformation and the force distribution on the airseal and the probable rate of material removal will vary. In fact, since the belt is spring-tensioned toward a flat configuration, excessive lingering of the fixtured blade at its stop position will flatten the airseal contour from that defined by the shoe. Thus, it would appear that the contour of the shoe needs to be adjusted by trial and error for a particular metal, belt, and operator, to achieve the desired dimensions.
Consequently, there is a need for an improved apparatus and method for restoring airseals which rapidly achieve the necessary contours in airseals while avoiding repetitive checking or trial and error in a setup.
As indicative of a further need, the typical turbine airfoil has two airseals. Most often one of these will have a greater radius of curvature than the other as it will be adapted to interact with a larger diameter section of the engine duct. Of course, these different radii are conveniently machined using the unattractive rotary grinder method, but there is a need for a lower cost method and apparatus for achieving acceptable contours in both airseals of a component being restored, without the obvious alternative of different setups for each seal.